Wilbert Pol's fork of mooneye-gb has some tests for LY=0. If these are correct, the behavior is that LY=153 lasts for just 4 cycles, then becomes LY=0, even though the mode is still VBLANK. So LY=0 actually lasts for 452 + 456 cycles. But it looks like the LY=LYC interrupt for LY=0 doesn't fire exactly when LY becomes 0, but after a 4-cycle delay. Another interesting case that is likely related: it seems that if you write 153 to LYC on the cycle when LY becomes 0, the STAT interrupt will fire.

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